Question: 9 notebooks cost $13.86. Which equation would help determine the cost of 7 notebooks?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 notebooks. We want to know the cost of 7 notebooks. We can write the numbers of notebooks as a proportion: $\dfrac{9}{7}$ We know 9 notebooks costs $13.86. We can let $x$ represent the unknown cost of 7 notebooks. The proportion of these costs can be expressed as: $\dfrac{\$13.86}{x}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{9}{7} = \dfrac{\$13.86}{x}$